There are commercially available telescope setup for this.
Ken the 18" you mentioned perhaps its this one Centurion 18.
http://www.astroworks.com/
Also this chap Johanes has done some amazing work with his scope.
http://panther-observatory.com/ see under Observatories - Telescopes. He has a 16" Cassegrain that can be used at Primary focus and Secondary focus. Its made by Philipp Keller, link below to some info on his scopes range from 400mm to 600mm with prime and secondary focus options.
http://www.astrooptik.com/Komplettgeraete/Cassf9_e.htm